1. Field of the Invention
The present invention relates to X-ray generating tubes applicable to diagnosis applications in medical apparatuses, non-destructive X-ray imaging in the field of industrial apparatuses, and the like. In particular, the present invention relates to transmissive targets applicable to the X-ray generating tubes.
2. Description of the Related Art
There have been demands for increasing the durability of X-ray generating apparatuses configured to generate X-rays in order to reduce the necessity of maintenance. One of the main factors that determine the durability of an X-ray generating apparatus is thermal resistivity of a target that is an X-ray source.
In an X-ray generating apparatus configured to irradiate a target with an electron beam to generate X-rays, the “X-ray generation efficiency” of the target is about 1%, so most of the energy input to the target is converted into heat. If the heat generated in the target is not adequately “released,” the target may be damaged by the heat load, which imposes restrictions on the thermal resistivity of the target.
It is publicly known to use a transmissive target including a target layer in the form of a thin film containing a heavy metal and a substrate configured to transmit X-rays and support the target layer in order to improve the “X-ray generation efficiency” of the target. Japanese Patent Application Laid-Open No. 2009-545840 discusses a transmissive target with an “X-ray generation efficiency” that is 1.7 to 2.0 times greater than those of the conventional rotating anode reflection targets.
It is publicly known to use a diamond as a substrate supporting a target layer of a layered target to promote the “release of heat” from the target to the outside. U.S. Pat. No. 6,850,598 discusses using a monocrystalline or polycrystalline diamond as a substrate supporting a target layer to improve the heat releasing property and decrease the focal spot. Having high thermal resistivity, high heat conductivity, and high X-ray transmittivity, diamonds are a suitable material for use as a support substrate of a transmissive target. Polycrystalline and monocrystalline diamonds have similar heat properties, but polycrystalline diamonds are advantageous in that they are inexpensively available.
Polycrystalline diamonds have physical properties similar to those of monocrystalline diamonds in terms of heat conductivity, thermal resistivity, and X-ray transmittivity as a support substrate for use in a transmissive X-ray target. Further, polycrystalline diamonds are more advantageous than monocrystalline diamonds in that millimeter-order sized support substrates can be supplied inexpensively and stably.
However, as a result of intensive and extensive studies conducted by the present inventors, it has been confirmed that when a transmissive target using a polycrystalline diamond as a support substrate is repeatedly operated as discussed in U.S. Pat. No. 6,850,598, a problem arises that X-ray outputs decrease or discharge occurs.